Questions of host-parasite strain compatibility, tissue specificity, and interactions lie at the core of scientific parasitology and are important in development of rational control methods for parasitic diseases. We are studying these relationships using the snail Biomphalaria glabrata, and cell cultures derived from it, together with Echinostoma paraensei. Miracidia can be converted to sporocysts in vitro, and second generation sporocysts dissected from snails can be maintained in cultures. Uptake and fate of labeled materials is followed by autoradiography. Antagonistic interaction (predation) by rediae of E. paraensei upon both mother and daughter sporocysts of S. mansoni has been demonstrated in culture and is being compared to similar events in vivo. Schistosome cercariae, cultured in vitro, are being tested for infectivity to rodents. Verification of molluscan identity is being attempted using serum from a rabbit immunized with B. glabrata embryo tissue. The surface of early sporocysts converted in vitro is being tested immunologically to determine whether there are reactive sites or areas indicating antigenic similarity to the molluscan host. These and associated studies will continue in the coming year.